I'm trying to help a friend with an 81 westy air cooled troubleshoot a problem.

When the van has 4 people and luggage, and has been driven an hour or
so, there is very noticable "clutch chatter" when starting out in
first gear.

The clutch feels fine the rest of the gears and the transmission feels
fine too.  When the van is cold and only one person it never seems to
chatter.

Any ideas?